Field of the Invention
The present invention relates to the optimization of the network structure of a radio communication system. More precisely, it relates to a method for assigning geographically oriented units of a given first hierarchical level of a radio communication system to geographically oriented units of at least one higher second hierarchical level.
Radio communication systems such as the GSM system have a hierarchical structure. The lowest level of this structure is formed by transmitter/receiver stations which communicate by radio with terminals, which in GSM terminology are denoted as base stations, or with the cells served by them. A plurality of base stations or their cells are supplied by a base station controller BSC. The totality of the cells of the base stations supplied by a BSC is therefore also denoted as a BSC region. A plurality of BSCs are supplied in each case on a higher hierarchical level by a mobile switching center (MSC). The totality of the cells assigned to an MSC is therefore also denoted as the MSC region.
In order, within an MSC region, to find a subscriber to whom a connection is to be set up, a search signal is emitted on the broadcast channel and causes the subscriber terminal to respond. When the number of the subscribers in an MSC region is low, this search signal can be emitted in all cells of the MSC region. When the number of the subscribers is large, the capacity of the broadcast channel is not sufficient for this purpose, and it can be necessary to subdivide this MSC region into a plurality of interconnected location areas and to emit the search signal only in that location area in which it is known that the targeted subscriber is located. On the other hand, the subdivision of the MSC region into location areas will cause the MSC to keep a record as to which of the various location areas of an MSC region a subscriber is currently located in. The processing load which is placed on an MSC by the management of the mobile subscribers is therefore a function of the type of division of the MSC region into location areas. The division of the MSC region or its location areas into BSC regions also influences the processing load. Consequently, there is a need for methods which, starting from a given distribution of base stations, MSCs and BSCs in a geographic zone, in each case permit the specification of assignments of the base stations to BSCs, to location areas and/or to MSCs which minimize the processing load, or permit the largest possible number of subscribers to be served for a given processing performance of the mobile radio communication system.
This processing load occurs predominantly in the MSCs. The MSC in a mobile radio communication system has two main functions, call processing and mobility management.
Call processing is understood here as the processing of any type of voice or data communication. The load caused at the MSC by call processing is a function of the type of communication (data, fax, short message service, etc.) and of the position of the two communicating subscribers relative to the network topology. It is, for example, different for calls within the mobile radio communication system and for calls from an external network into the mobile radio communication system or from the mobile radio communication system into an external network. In the case of calls within a network, as well, the load at the MSC is different depending on whether the two subscribers of a call belong to the same BSC and/or MSC region or not.
Mobility management is understood to be all transactions in the system which are caused by the tracing and recording in that cell of the network in which an individual subscriber is located. A distinction is to be made here between handover and updating the location of a subscriber (location update).
Handover is understood as the change of a subscriber from one cell into another cell in simultaneous conjunction with maintaining a running connection by allocation of resources to the other cell. The load which is caused by a handover at the MSC depends substantially on the position of the two cells participating in the handover relative to the network topology. Three cases which load the MSC to a different extent in each case, are to be distinguished here:
a. both cells belong to the same BSC region, but not to the same base station;
b. they belong to the same MSC region, but not to the same BSC region, or
c. they do not belong to the same MSC region.
In the case of the GSM network, a handover between cells which belong to the same base station need not be taken into account, since the MSC does not participate in the management of such a handover.
A location update takes part when a subscriber changes his cell in the stand-by mode. Two cases are to be distinguished here:
when the two cells belong to the same location area he need not be acknowledged by the MSC, nor does he cause a load there, and
when said subscriber changes the location area (or the MSC region), this subscriber must be removed from one list in the MSC and entered into another (in the same or another MSC) depending on whether the two cells belong to two different location areas within an MSC region or to two different MSC regions.
The actual load which is caused at the MSC by these diverse management operations, and which can be measured, for example, in the form of required computing time or the number of processor commands executed, can vary for different models of the MSCs.
However, the optimization of network topologies is complicated not only by the different types of modes to be taken into account, but also by virtue of the fact that the individual cells, BSC regions, MSC regions etc., in general terms the various geographically oriented units of different hierarchical levels, differ in their properties such as, for example, number of subscribers, subscriber behavior etc.
All of this renders optimization of a network structure an extremely complex problem to solve which use has been made to date essentially of empirical rules or heuristic approaches.
It is accordingly an object of the invention to provide a method for assigning geographically oriented units of a given first hierarchical level of a radio communication system to geographically oriented units of at least one second, higher hierarchical level which overcomes the above-mentioned disadvantageous of the prior art apparatus and methods of this general type. In particular, it is an object of the invention to provide such a method that permits effective minimization of the processing outlay connected with the management of calls and subscriber mobility, in which the method is based on mathematical foundations, and requires a low computational outlay.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for assigning geographically oriented units of a first hierarchical level of a radio communication system to geographically oriented units of at least one second hierarchical level that is higher than the first hierarchical level.
A first step in the method is to set up functions which specify, as a function of the number of subscribers of the radio communication system, the size of a processing load which is caused by a geographically oriented unit of the first hierarchical level at a node of the radio communication system such as an MSC for example. A basis for setting up these functions can be taken from empirical measurements of the traffic volume and of the subscriber behavior in the individual cells of the radio communication system.
Starting from these functions, a formula is then set up which permits these functions to be used to calculate the processing load occurring at the relevant nodes for a given assignment of the units of the first hierarchical level to the units of the second hierarchical level.
Using this formula, which supplies the load as a function of the number of subscribers in the network, it is possible to select an assignment which permits the greatest possible growth in the number of subscribers above the current number of subscribers in the network, without the load to be processed by a node in the radio communication network exceeding the resources of this node.
Different methods can be used to select this assignment.
In accordance with an added feature of the invention, methods of linear optimization are preferably used to select the assignment. Such methods are described in various text books and implemented in a majority of commercially available computer programs.
In accordance with an additional feature of the invention, the method can be used on different hierarchical levels of the radio communication system. Thus, for example, it is possible to select cells as units of the first hierarchical level and BSC regions of the radio communication system as units of the second hierarchical level, in order in each case to optimize the structure of the system within the location area or MSC region. It is also possible for location areas and/or BSC regions to be taken as units of the second hierarchical level, in order to optimize the structure of the overall network.
In accordance with another feature of the invention, in order to keep the processing simple, the functions which specify the size of the load can be approximated as linear functions of the number of subscribers.
In accordance with a further feature of the invention, a substantial simplification of the method is provided. This can be achieved by starting from a given, typically actually existing assignment of the units of the first hierarchical level of the radio communication system to the units of the second hierarchical level, and when selecting the assignment which permits the largest possible rise in the number of subscribers, by taking account only of assignments which differ from the given assignment only in the case of such units of the first hierarchical level as are respectively situated in the given assignment at the boundaries between two units of the second hierarchical level. This mode of procedure is particularly expedient in radio communication systems having a large number of units of the first hierarchical level, since the number of the theoretically possible assignments of units of the first hierarchical level to the units of the second one increases super-exponentially with the number of these units, and the number of assignments possibly to be taken into account is reduced radically in this way.
Of course, the consequence of such a limitation can be the failure to find an even better distribution which would have required the redistribution not only of geographically oriented units of the first hierarchical level situated at a boundary, but also of a unit adjacent thereto and situated in the interior of a unit of the second hierarchical level. However, this is not a serious disadvantage since, after a single pass of the method, such a unit of the first hierarchical level comes to be situated at the boundary of the newly formed units of the second hierarchical level, and can therefore likewise be reordered by iterative application of the method.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in an optimization of the network structure of a radio communication system, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.